Method of electrodeposition of iron



April 11, 1950 R, CNN 2,503,235

METHOD OF ELECTRODEPOSITION OF IRON Filed March 13, 1946' lrzyanlor Ja/zzz R Kai/z Patented Apr. 11, 1950 METHOD OFELECTRODEPOSITION F mom John R. Cain, South Strafford, Vt., assignor to Sulphide Ore Process Company, Inc., New York, N. Y., a corporation of Delaware Application March 13, 1946, Serial No. 654,077

The present invention relates to the electrodeposition of iron.

In the art of the electrodeposition of iron, there are two well-defined, substantially mutually exclusive procedures. One is predicated upon the use of a soluble anode which goes into solution in the electrolyte, upon the passage of the electric current, while iron deposits upon the cathode as a free metal. The other procedure is predicated upon the use of an insoluble anode, which therefore does not dissolve and is substantially unaffected, during electrolysis, and in which, accordingly, the metal to be deposited on the cathode must be supplied to the electrolyte already in solution.

Of these procedures, the one employing a soluble anode entails the introducton of insoluble impurities which cumulatively contaminate the electrolyte of the cell, unless pure iron anodes are used. The use of pure iron, however, is practically prohibitive because the purpose of the production of electrolytically deposited iron is primarily to obtain a pure iron as the product. If the anode were already composed of pure iron, the only conversion to be effected by the process would be that of the physical form of the iron which can usually be efiected more readily and economically by mechanical means.

The procedures of employing insoluble anodes, on the other hand, avoid contamination of the electrolyte or of the cell during electrolytic deposition of the iron, because the anodes are completely resistant to both the electrolyte and the electric current. However, in such procedures, the tendency is for the electrolyte adjacent to the anode to become more and more con-centrated with ferric iron in the form of various ferric salts. The electrolyte adjacent to the cathode becomes more and more concentrated with ferrous iron in the form of ferrous salts. Moreover, along with these tendencies the potential of the electric current is such as to favor liberation of hydrogen near the cathode and acid radicals or oxygen near the anode. As a result of the latter, side reactions may occur with segregation of gaseous, liquid, or solid by-products which interfere with the normal continuous electrolytic deposition of iron by the cell.

. To counter-balance or offset these progressive reactions and influences it is the present practice of the art to interpose a porous diaphragm between the anode and the cathode, when employing insoluble anodes. Usually such diaphragms take the form of acid resistant, anode bags, made of asbestos or the like, and serve to maintain the 1 Claim. (Cl. 204-113) electrolyte surrounding the cathode, or catholyte, at a higher level than the electrolyte surrounding the anode or anolyte, so as to prevent migration of the anolyte into the catholyte. On the contrary, they positively promote the migration of the electrolyte from the catholyte compartment into the anolyte compartment.

Moreover, it is frequently practiced, in such procedure, to withdraw the anolyte from the anode compartments, to dissolve a fresh supply of iron therein, thus reducing the ferric iron content to ferrous condition, and then to return the reduced ferrous iron solution to the catholyte compartment. This has the effect of maintaining the difference in surface levels and hence the pressure of the catholyte above that of the anolyte, thus directing the flow of the electrolyte through the porous diaphragm or anode bag from the cathode to the anode. The purpose as well as the result of such circulation is to maintain the catholyte in ferrous condition and free from ferric iron.

However, it is now found contrary to the prior belief and practice of the art, that it is not essential nor in fact desirable for best electrolysis to maintain the electrolyte surrounding the oathode completely free from ferric iron and ferric salts. It is found that, when employing an insoluble anode, it is not only effective but preferable to have a considerable amount of ferric iron or ferric salts in solution in the electrolyte surrounding the cathode. In consequence of this discovery and as a part of it, it is also found that the provision of an intervening porous diaphragm may be entirely dispensed with, and that all of the inconvenience of construction, manipulation and control of two separated electrolytes, which the use of such diaphragm entails, can also be eliminated.

It is found that an electrolyte containing ferrous iron, in the form of a dissolved ferrous salt, and approximately .l% to 3% the ferrous iron content in ferric condition in the form of a dissolved ferric salt, and acidified to a pH value below 2.0, is especially advantageous, not only for the electrodeposition of iron but also for the selective dissolution of iron, in the preparation of fresh electrolyte directly from the raw materials which are usually in relatively crude, impure condition.

Such electrolytic iron solutions are furthermore found to be competent to perform this complete cycle of operations, continuously and substantially without consuming the free acid initially provided to adjust the pH value.

These results may be assured by a rapid circuthe ferric iron has been reduced to ferrous con dition. If such separation is effected, it will also be before the free acid contenthas reacted appreciably upon the ore and thus been neutralized. Thereby the free acid content and concentration of the electrolyte is maintained. The condition of the electrolyte is thus stabilized and preserved through repeated cycles of such operations by withdrawing it from the cell, leaching, and returning it promptly and preferably continuously into the electrolytic cell. It also presents an advantage to return the electrolyte adjacent to a cathode, to circulate it alternately and rapidly over the cathodes and anodes of the cell, successively, and then to withdrawit from the cell at a point adjacent to the final anode of the cell.

If the free acid of the. electrolyte is volatile, and electrolysis is conducted at elevated temperatures, as for the production of ductile electrolytic iron sheets, some of the acid may be volatilized and lost from the'solution even though decomposition does not take place. In such cases it may be necessary, periodically or continuously, to make up such losses by suitableaddition of the free acid. This may be effected by introducing the acid directly into the electrolyte in the cell or into the electrolyte which has been withdrawn from the cell and used to leach the iron sulphide ore. Preferably, however, it is added to the electrolyte'or leachliquor asit comes from the sulphide ore and after it has been filtered or otherwise treated to remove undissolved or separated solid matter, such as free sulphur.

A typical example of procedure in accordance with the invention will be described asconducted with an electrolyte made 'upof ferrous chloride, ferric chloride and hydrochloric acid, as representative of ferrous and ferric'salts and free acid, respectively, and with reference to the accompanying drawing which is a diagrammatic illustration of suitable apparatus and of the several operations.

I To commence the operation, an aqueous solution of ferrous chloride (which may contain for example, 80 to 120 grams of iron per liter) is prepared and supplied to head tank I. Itis also preferably acidified to a pH value considerably below 2.0 by the addition of free hydrochloric acid. From this head tank I, the solution may be led by the pipe 2 into the electrolytic cell 3, comprising one or more anodes 4, 4, preferably vertically mounted on the bottom of the cell and a corresponding series of cathodes 5, 5, suspended vertically in thecell from bus bars 8, 6 and preferably standing above the bottom of the cell. In that way the electrolyteintroduced into the tank or cell from pipe 2 will tend to flowfirst under a cathode 5, thence over an anode 4, under the next cathode 5, etc., to the end of the cell. The cell may also be supplied with a submerged heating coil 1, for steam or hot water, to maintain an elevated temperature in the electrolyte. At the end of the cell, remote from the inlet of pipe 2, and adjacent to the last anode, is provided an outlet 8, leading to pump 9, which,

in turn, is adapted to deliver the electrolyte through pipe I0 into an upper leaching tank II. This tank is provided with a mixing paddle 12, with suitable motive power for its rotation (not shown) and also with means I3 for introducing finely divided ferrous iron ore, such as pyrrhotite, for example, therein.

One convenient form of the type of ferrous ores which may be used is a ferrous sulphide ore of the pyrrhotitic type obtained from the chalcopyrite-pyrrhotite ores of the Appalachian region, for example. Thi may be crushed and ground, to a sufficient degree of fineness (e. g. 10 mesh and finer) and the chalcopyrite then separated from the pyrrhotite, by flotation. The pyrrhotite is obtained as the concentrate, by such operations, and may be delivered for use in the present process in either wet or dry condition.

t is preferred, however, that it should not be allowed to oxidize, by prolonged exposure to the atmosphere before such use, for in such case it is subject to oxidation to sulphate and other oxygen compounds of sulphur which may interfere with its complete dissolution and uniformity of electrolysis and electrodeposition in the cell.

This ore may therefore be introduced into the leaching tank II, in either wet or dry condition, a desired, as indicated at I3.

The electrolyte in the cell 3 may be withdrawn almost as soon as the electrolytic current has been set up and electrodeposition of metal has commenced. It is advantageous, however, to Wait until the operation of the cell has proceeded for a sufiicient time to oxidize the ferrous chloride to ferric chloride, to the extent of approximately 0.1% to 2.0% of the iron content of the electrolyte.

In this condition-namely, of an electrolyte of ferrous and ferric chlorides, in solution in the proportions of about the ratio of parts ferrous iron to .1 to 2 parts of ferric iron-and containing free hydrochloric acid, manifesting a pH value below 2.0, is preferred not only for operation of the electrolytic cell, but also for the dissolution of ferrous sulphide ores of the pyrrhotite type.

The electrolyte therefore (preferably when it contains the upper proportion of its iron con.- tent which is desired, as ferric iron) is delivered by the pump 9, into the tank H, vhere it is brought into contact with the iron ore to be dissolved. The ore is present or introduced in such manner as to maintain an excess, and the stirrer l2 operated so as to agitate the mixture vigorously, thus maintaining the solids in suspension and effecting immediate and maximum contact and reaction between the component solids and liquids.

The sludge may be withdrawn after a brief contact and partial reaction and dissolution of theme, through'the draw-off cock I4 and pipe I5 into a second leaching tank I6 which is also provided with a stirring paddle I? driven by a motor or the like (not shown). This maintains the agitation and reaction of the mixture. Heater 28 also maintains the temperature.

The concentration of the leach liquor in tank I6 may be adjusted with respect to its ferrous chloride concentration or hydrochloric acid concentration, or both, by letting in make up solution, periodically or slowly and continuously, from the head tank I, through pipe I 8 controlled by the valve I9. This may be done during the leaching operation, if desired,or after the desired degree of reduction of the ferric chloride in solution to ferrous chloride, in solution, has been effected, without formation of insoluble residues or byproducts such as ferrous hydroxide, ferric oxychlorides or the like, but if these should have been incipiently formed the addition of fresh, acidified ferrous chloride solution is effective to dissolve them. It is, of course, better to prevent their formation.

Likewise, if the ferric chloride has been considerably reduced to ferrous chloride, so that more ferric chloride is desirable in the second leaching tank IS, the electrolyte from the cell 3 may be delivered directly thereto by the pump 9 through pipe 20 and valve 2|. Such double or repeated contact-of the ferrous sulphide ore with the ferric chloride electrolyte as a'leaching liquor promotes the dissolving action of the solution upon the ore and the disintegration of the ore and also the separation of the sulphur therefrom in the form of free sulphur and in a condition suitable for effective separation and use as a valuable by-product. It is especially effective and desirable upon pyrrhotitio iron sulphide ores which have been allowed to age or weather and thus become partially oxidized. Such ores, due to the segregation of free sulphur, formation of superficial deposits of salts or other by-products, or the like action, may tend to become somewhat less reactive or slower in going into solution in the electrolyte than the fresh ores and for this reason require a longer time or more agitation or both, to effect their complete solution. The entire iron content of the iron ore delivered to the second leaching tank is thus taken into solution, substantially without leaving any valuable residues therein.

The leach liquor may be separated from any residues which may be left by withdrawing it through the overflow 23 at the top of tank l6 and then passed through the filter 24, which collects the free sulphur from the dispersion therein. The clear filtrate of adjusted ferrous chloride: ferric chloride ratio of about 100 to 1 and hydrochloric acid (at a pH below 2.0) is led through pipe 25 to the cell 3, as required and regulated by valve 26.

In operation, therefore, the filtered leach liquor will be of increased concentration of ferrous chloride, of decreased relative concentration of ferric chloride, e. g. to 0.1% to 1% of the total iron concentration of the electrolyte, and of substantially the same hydrochloric acid concentration and pH value. If free metallic iron should be present in the ore the hydrochloric acid may become slightly neutralized thereby with the formation of ferrous chloride. Some of the free acid may also be lost by volatilization. But the pyrrhotitic iron is first and primarily reacted upon by the ferric chloride, in accordance with a representative equation as follows:

(11:5 to 16 per Danas A Textbook of Mineralogy) This reaction is favored and the leaching action preferably restricted thereto by arresting the reaction before complete reduction of the ferric chloride present and effective so to react. If this is done the free hydrochloric acid in the leaching liquor does not react upon the iron ore. But otherwise it will so react (i. e. if all of the ferric iron is reduced to ferrous iron condition) with liberation and loss of sulphur as hydrogen sulphide gas as follows:

When the electrolysis is conducted at elevated temperatures, the hot electrolyte, as withdrawn, is additionally effective to react upon and dis solve the sulphide iron ore and to hold the resulting iron salts in solution. It also tends to separate the free sulphur in fine colloidal form, and to promote its subsequent coagulation into a curdy form which is quickly and completely separated out by filtration, without clogging the filter.

It may be desirable and advantageous, to have the electrolyte first contact the ore, by leading it into the vertical tube 21, in the tank l6, which opens at its lower end adjacent to the stirrer and to the bottom of the tank. In this way the incoming ferric chloride solution contacts first with the ore at or near the bottom, without stirring up or mixing with the upper levels of supernatant reacted liquor which contains primarily ferrous chloride and colloidal sulphur. The leach liquor may thus be controlled in its composition as it rises and passes off through the overflow and is returned to the electrolytic cell for continued electrodeposition of its iron content.

By such procedure, and especially by rapid cycling and re-cycling of the electrolyte through the electrolytic cell and leaching and filtering operations, the electrolyte and the electrodeposition of iron therefrom is stabilized and maintained substantially constant in respect of the ferrous chloride concentration, the ferric chloride concentration, the ratio of the one to the other, the aggregate concentration of both, the temperature of the electrolyte, and the hydrochloric acid concentration and pH value, the rate of deposition of iron therefrom, and the consequent character of the iron deposit.

The present procedure also avoids the consequences of variations in these factors, the loss of electric current efficiency and uniformity of operation due to porous diaphragms, such as anode bags, the formation of insoluble compounds in the electrolyte in the cell, emission of chlorine, and the difliculties and inefficiencies of dissolving ores in ferric solutions so as to produce solutions of ferrous salts, substantially free from ferric salts and of a pH value higher than 2.0.

This procedure also economizes in the heat requisite for the operation as compared with the procedures in which prolonged treatments or settling of the electrolyte is required for separation of solids and segregations of suspensions such as colloidal sulphur.

This latter, as well as the segregation and separation of dissolved copper, if present in the electrolytes, may be effected by subjecting the electrolyte to contact with some of the acid soluble sulphide ore, such as pyrrhotitic iron sulphide, and subjecting to temperatures approximating the boiling point, for example to C., or by active boiling of the electrolyte, followed by filtration.

I claim:

Method of electrodeposition of iron from a non-diaphram cell using an insoluble anode, comprising passing an electric current through an aqueous electrolyte solution consisting essentially of ferrous chloride, maintaining the electrolyte in the ratio of 100 parts ferrous iron as ferrous chloride to .1 to 3 parts ferric iron as ferric chloride with free hydrochloric acid in amount not to raise the pH above pH 2, continuously withdrawing the electrolyte from the cell and contacting it with a pyrrhotitic ore to reduce the concentration of ferric iron to 0.1% to 1% of the total iron concentration of the electrolyte and increasing the concentration of ferrous chloride, filtering the leach liquor and continuously returning same containing the increased concentration of ferrous chloride, the decreased concentration of ferric chloride and with substantially the same hydrochloric acid concentration and pH value to the cell, the circulation of the electrolyte being at a rate to maintain the above-described concentration thereof substantially constant.

JOHN R. CAIN.

REFERENCES CITED The following references are of record in the file of this patent:

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